1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having a lower flow mixing chamber at the entrance to the fuel rod bundle and water cross which minimizes maldistribution of flow entering the separate mini-bundles making up the fuel rod bundle.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. Examples of such fuel assemblies are illustrated and described in U.S. Pat. Nos. 3,689,358 to Smith et al and 3,802,995 to Fritz et al and Canadian Pat. No. 1,150,423 to Anderson et al, as well as in the patent applications cross-referenced above.
In a fuel assembly of this type the fuel rods in the central region of the bundle thereof may be undermoderated and overenriched. In order to remedy this condition by increasing the flow of moderator water through this region of the assembly, several arrangements have been proposed. In the Fritz et al patent, one or more elongated empty rods are substituted for fuel rods in the central region of the assembly. In the above cross-referenced Olson et al patent application, water tubes are arranged in a cross-like pattern among the fuel rods in the assembly. In the Anderson et al patent, an elongated centrally-disposed stiffening device with vertical water passageways is used in the assembly. In the above cross-referenced Barry et al, Doshi and Lui patent applications, an elongated centrally-disposed water cross is used in the assembly.
As disclosed in the aforementioned latter three cross-referenced applications, the central water cross has a plurality of four radial panels which together form a cruciform water flow channel which divides the fuel assembly into four, separate elongated compartments, with the bundle of fuel rods being divided into mini-bundles disposed in the respective compartments. The water cross thus provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of, but separated from, adjacent fuel rods in the mini-bundles thereof.
The fuel rods of each mini-bundle extend in laterally spaced apart relationship between an upper tie plate and a lower tie plate and connected together with the tie plates comprises a separate fuel rod subassembly within each of the compartments of the channel. A plurality of grids axially spaced along the fuel rods of each fuel rod subassembly maintain the fuel rods in their laterally spaced relationships. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof.
As mentioned initially, coolant is passed along the fuel rods for removing heat therefrom. In the design of BWRs in the United States, subcooled water enters the bottom nozzle of the fuel assembly through a side entrance. Thereafter, the water is distributed upwardly into the four mini-bundles and the water cross. Typically, flow through the water cross is approximately 9 to 10 percent of flow through the fuel bundle. Due to the side entry characteristics of these BWRs, it has been found that significant maldistribution of flow results, That is, the difference in the mini-bundles entraining the highest and lowest mass flow rates is about 15 percent.
Maldistribution of flow can lead to several problems. First, an overall degradation of bundle critical power ratio (CPR) margin can result. Since water flowing in the mini-bundles cools the fuel rods thereof as it flows along their heated surfaces, the amount of water entering each mini-bundle determines its CPR characteristics. However, the overall bundle CPR margin is dictated by the CPR margin of the most limiting mini-bundle. Known relationships indicate that a 15 percent reduction in the mass flow rate in one of the mini-bundles leads to a CPR margin degradation of about 7 percent which is potentially quite problematic.
Second, since ventilation holes or flow communication openings have been provided at the outer vertical edges of the watercross panels at several axial locations, the 15 percent flow maldistribution between the mini-bundles also leads to transverse pressure gradients, causing cross-flow jets of water. Depending on these gradients, the jets could conceivably cause structural vibrations. The presently existing unavailability of adequate analytical/computational tools to evaluate the thermal-hydraulic-mechanical impact of such a situation leads to further uncertainies in estimating mini-bundle CPR margins, with consequent operational and licensing problems.
Third, while flow boiling typically starts after the first few feet of heated length in a BWR bundle, the presence of flow maldistritution of the magnitude mentioned above would tend to cause or initiate the boiling process at different axial locations in each mini-bundle. This disparity causes a net disparity in the amount of void generated for each mini-bundle which indicates improper neutron utilization or moderation.
Consequently, the need exists for further improvement of the BWR fuel assembly so as to eliminate or significantly minimize mini-bundle inlet flow maldistribution and thereby avoid the undesirable effects which accompany this condition.